This invention relates to a pattern-matching sewing machine, for sewing two sheets, such as cloths, each bearing the same patterns with the patterns matching.
Published Unexamined Japanese Patent Application No. S60-153896 (which corresponds to the U.S. Pat. No. 4,612,867, and the German Patent Application DE 33 46 163 C1) discloses a pattern-matching sewing machine of this type. In this machine, a pair of photo-sensors are placed before the sewing point to generate intensity data representing the brightness of the patterns on the two cloths. When the two cloths are fed for sewing, the mismatch distance of the patterns on the two cloths is detected using the intensity data, and the feed amount for at least one of the two cloths is gradually adjusted according to the mismatch distance to maintain the pattern match.
However, even if feed amount is given to the cloths based on the calculated mismatch distance, some species of cloths have hard-to-match patterns. This is because the set feed amount of the feed dog may not be the actual feed amount of the cloths, or because the actual feed amount changes according to various sewing conditions, even if the set feed amount is constant.
FIG. 17 shows the data for explaining the above situation where: two superposed cloths are sewn 450 mm under the conditions that lower feed amount of a lower feed dog is fixed at 2 mm per feed, while upper feed amount of an upper feed dog is varied, and then the mismatch distance between the two cloths is measured. Four kinds of cloths, (i.e., thick woolen cloth A, normal woolen cloth B, thin woolen cloth C, and non-woolen thin cloth D are measured. The mismatch distance of A, B, C, and D becomes zero when their upper feed amounts are set at 1.32 mm, 1.48 mm, 2.12 mm, and 2.88 mm, respectively. The slope of the curve in the vicinity of zero mismatch distance is largest in D, followed by C, B, and A. Therefore, in the case of the cloth D, a slight change in the upper feed amount increases the mismatch distance.
The difference between the preset upper-feed amount and the actual feed amount varies according to the species of cloths. For one cloth, a long-distance sewing is necessary to return the mismatch distance to zero as shown in FIG. 16A, and for another cloth the mismatch distance fluctuates, as shown in FIG. 16B. Thus the quality of the sewn products is unstable.
Further, even when the same species of cloths are sewn, the difference between the set upper feed amount and the actual upper feed amount is also caused by differences in: rotation speed of a main motor, shape of an upper feed dog or a presser foot, or pressure of the presser foot.